Astron. Nachr./AN330,No.1,100–106(2009)/DOI10.1002/asna.200811132 Edge-ondiskgalaxiesintheSDSSDR6:Fractionsofbulgelessandother disk galaxies S.J.Kautsch⋆ DepartmentofAstronomy,UniversityofFlorida,211BryantSpaceScienceCenter,Gainesville,FL32611-2055,USA 9 Received2008Sep29,accepted2008Nov17 0 Publishedonline2008Dec28 0 2 Key words galaxies: fundamental parameters – galaxies: irregular – galaxies: spiral – galaxies: statistics – galaxies: n structure a J The aim of this study is to determine the fractions of different spiral galaxy types, especially bulgeless disks, from a completeandhomogeneoussampleof15127edge-ondiskgalaxiesextractedfromthesixthdatareleasefromtheSloan 3 Digital Sky Survey. The sample is divided in broad morphological classes and sub types consisting of galaxies with 1 bulges,intermediatetypesandgalaxieswhichappearbulgeless.Asmallfractionofdiskyirregularsisalsodetected.The morphological separation is based on automated classification criteria which resemble the bulge sizes and the flatness ] A of the disks. Each of these broad classes contains about 1/3of thetotal sample. Using strictcriteriafor selecting pure bulgelessgalaxiesleadstoafractionof15%ofsimplediskgalaxies.Wecomparethisfractiontoothergalaxycatalogs G and find an excellent agreement of the observed frequency of bulgeless galaxies. Although the fraction of simple disk . galaxiesinthisstudydoesnotrepresenta“cosmic”fractionofbulgelessgalaxies,itshowsthattherelativeabundanceof h puredisksiscomparabletootherstudiesandoffersaprofoundvalueofthefrequencyofsimpledisksinthelocalUniverse. p Thisfractionofsimpledisksemphasizesthechallengeforformationandevolutionmodelsofdiskgalaxiessincethese - o modelsarehardpressedtoexplaintheobservedfrequencyoftheseobjects. r t (cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim s a [ 1 Introduction arelocatedingroupandlow-densityenvironment(Kautsch, 1 Gallagher & Grebel 2008). Hence, this type of environ- v 4 Simple disk galaxiesareflat, late-typedisk galaxiesofthe ment should preserve the bulgeless shape of those galax- 9 morphologicalHubbleclass∼Sdandlaterwithoutabulge ies. The contrary is observed as mergers are the domi- 8 component(e.g., Goad & Roberts 1979, 1980;Karachent- nant interaction process in groups (Barnes 1985) and the 1 sev 1989;Karachentsevetal. 1992;Kautsch etal. 2005b). group environment is effective in transforming morpholo- . 1 The formation and evolution of such thin galaxies is not gies(Kautschetal.2008b;Tranetal.2008).Minormergers 0 yet well understoodin the frameworkof Λ cold darkmat- already lead to bulge growth (e.g., D’Onghia et al. 2006; 9 termodels.Simulationshavedifficultiesinproducingdisk- Cox et al. 2008b; Kazantzidis et al. 2008) and the heat- 0 : dominatedand bulgelessgalaxies.The simulateddisksare ingofthethin,stellardisk(Purcell,Kazantzidis&Bullock v smaller,denserandhavelowerangularmomentumthanob- 2008). Near equal mass mergers can take place across all i X served,knownastheangularmomentumproblem.Adding different galaxy environments where disk-dominated and r feedbackprocessestothesimulationscanimprovethecre- simplediskgalaxiesreside(Karachentsev,Karachentseva& a ation of disk galaxies to some extent (e.g., Okamoto et Parnovskij 1993; Kautsch, Grebel & Gallagher 2005) and al. 2005;Scannapiecoet al. 2008),but D’Onghia& Burk- affect the overwhelmingmajority of Milky Way-sized ha- ert (2004), D’Onghia et al. (2006), Ko¨ckert & Steinmetz losasshowninmerger-treestatisticswithinΛCDMN-body (2007)andPiontek & Steinmetz(2008)showthatproduc- simulations(Stewartetal.2008).Itisassumedthat∼70% ing the observed structural and kinematical properties of of these Milky Way-sized halos contain disk-dominated disk-dominated galaxies and simple disks cannot be im- galaxiesand∼11%ofthehalosarethehostsofsimpledisks provedbyaddingfeedbackprocessesandbyincreasingthe (Stewart et al. 2007, and referenced therein). Undergoing numericalresolutionofthesimulations.Therefore,thefor- suchamergereventisbasicallyinfaustforadisk-dominated mationmechanismsofsimpledisksremainenigmatic. galaxyandespeciallysimpledisks,i.e.,N-bodysimulations of mergers exhibit that the stellar disk can be – but is not Duringandaftertheirformation,disk-dominatedgalax- always (Koda, Milosavljevic & Shapiro 2007; Hopkins et ies and simple disks are very sensitive to various pro- al.2008)–completelydisruptedandmorphologicallytrans- cesses that are responsible for transforming those objects formedintoanearly-typegalaxy(e.g.,Toomre1977;Stein- into bulge-dominated galaxies or even destroy the disks. metz2003;Cox&Loeb2008). The majority of disk-dominated and simple disk galaxies Socalled“pseudobulges”cangrowduetointernaldisk ⋆ Correspondingauthor:[email protected]fl.edu instabilities. In this secular evolution model gas sinks into (cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim Astron.Nachr./AN(2009) 101 the disk center and the stars from a subsequent central morphological separation criteria. This allows us to study star-formationperiodformabulge(Kormendy&Kennicutt thefractionsofdifferentedge-ondisksinamorerobuststa- 2004;Kormendy&Fisher2005).Galacticbarssupportthe tistical manner. The galaxies in this new sample are also gasflowtowardsthegalacticcentersandthusareimportant divided into morphological classes using a new approach for secular evolution. Bars are frequently detected in bul- comparedtothecatalog.Theimprovedobjectdetectionand gelessgalaxies(Barazza,Jogee&Marinova2008),making especiallythe muchlargercoveragearea inthe skyshould simplediskgalaxiespotentialcandidatesforongoingsecu- provideupdatedstatistics,accuracyandhomogeneityofthe larevolution. fractionsof disk-dominatedgalaxiesin the localUniverse. The first comprehensive catalog of edge-on disk- Inaddition,weprovideauniquecomparisonoffractionsof dominated galaxies is the “Flat Galaxy Catalog” (FGC, bulgeless disk galaxies from several catalogs and derive a Karachentsev et al. 1993) and its extension, the “Revised robust estimation of the frequency of simple disks among Flat Galaxy Catalog” (RFGC, Karachentsev et al. 1999). spiralsinthelocalUniverse.Theknowledgeofthisfraction FGC and RFGC are optical all-sky surveys. RFGC con- iscrucialforstudiesabouttheformation,evolutionandsur- tains4236visuallyselected“flat”galaxies.Acollectionof vival of disk-dominatedgalaxies. Therefore,this work de- disk-dominatedgalaxiesin the near-infraredis gatheredin liversusefulstatisticalresultsforseveralfollow-upstudies “The2MASS-selectedFlatGalaxyCatalog”(Mitronovaet suchasthefractionofsimplediskgalaxiesathighredshifts. al.2004). This article has the following structure: in Sect. 2 we In order to contribute to a better characterization of describehowedge-ondiskgalaxieswereselectedinprevi- flat galaxies, Kautsch et al. (2005b; 2006a) carried out a ousworksandinthispresentstudy.Thequantifiedmorpho- workwhichcompiledauniformsampleofdisk-dominated logicalclassificationofgalaxieswithandwithoutbulgesis galaxies in the optical wavelengths from the Sloan Dig- discussed in Sect. 3. The results of this study in terms of ital Sky Survey (SDSS, York et al. 2000). The SDSS is numbersandfractionsandacomparisontothefractionsof ideal for the identification of such galaxies with its deep, recently publishedstudies are shown in Sect. 4. In Sect. 5 multi-wavelength, homogeneous and large-area coverage. wesummarizetheresults. They analyzed SDSS data from the Data Release 1 (DR1, Abazajian et al. 2003) and compiled a catalog of 3169 2 Data acquisition galaxies with prominent edge-on stellar disks (Kautsch et al. 2006a, hereafter “the catalog,” which is accessible on- line1 and Kautsch et al. 2006b). Using an automated al- In the catalog, edge-on disk galaxies were selected based gorithm, galaxies in the catalog are divided into galaxies on selection criteria that are similar to those of the orig- withbulges,intermediatetypesandsimplediskgalaxiesand inal approach by Karachentsev et al.’s catalogs FGC and subclasses. 15.8% of the catalog galaxies are found to be RFGC.TheoriginalobjectselectionbyKarachentsevetal. simpledisks.Thisdemonstratesthatbulgelessgalaxiesare (1993;1999)isbasedonthevisualidentificationofgalax- frequent,especiallyamongintermediate-massstar-forming ies with an axial ratio a/b ≥7 and a major axis diameter galaxies(Matthews&Gallagher1997).Thisfrequencyin- of >∼40′′ in the blue band POSS-1 copies and ESO/SERC creases up to 1/3 of the catalog galaxies when puffy disks photographicplates.Inordertocomeascloseaspossibleto are included. The boundaries between the galaxy types in these originalvalues, a training set of RFGC galaxies that the catalog are not sharp, suggesting simple disks are the wererecoveredintheSDSSDR1wasanalyzedinthecata- faintendinacontinuumofdiskgalaxyproperties,e.g.,sur- logwithrespecttotheirsizes,magnitudesandcolordistri- facebrightness. bution.ThisallowedKautschetal.(2006a)totranslatethe Since DR1, SDSS has undergone significant changes. originalobjectselectionfromtheRFGCintoaquerythatse- DR1 provides a survey area of 2099 deg2 of imaging lectsallgalaxiesbrighterthanmg =20fromtheDR1“Best data. The data releases DR2 (Abazajian et al. 2004), DR3 GalaxyTable”withaxialratios>3andamajoraxisdiame- (Abazajian et al. 2005), DR4 (Adelman-McCarthy et al. ter>30′′withincertainbroadcolorlimits.Forthedetailsof 2006), DR5 (Adelman-McCarthy et al. 2007) and DR6 thisquery,pleaseconsultKautschetal.(2006a;2006b).Our (Adelman-McCarthy et al. 2008) are now available. DR6 intention now is to collect edge-ongalaxiesfrom the DR6 covers 9583 deg2 of imaging in total and has an r-band in the same reproducible fashion. Therefore we apply the depth of approximately 22.2 mag. Hence, DR6 contains SDSS CasJobs on the SDSS Context DR6 to the “Galaxy morethanfourtimesthefootprintareaofDR1.Inaddition, Table”.CasJobs2isanonlineinterfacethatperformsqueries changesweremadebetweenDR1andDR2concerningthe onvariousSDSSdatasetsusingthe“StructuredQueryLan- deblending of overlapping objects. This correction should guage” (SQL) and the “Galaxy Table” is the table of the improvethedetectionofindividualgalaxies. SDSS data archive that contains all parametersfor objects In this study we collect edge-on disk galaxies as done selected as galaxiesin DR6 with the highestquality at the inthecatalog(Kautschetal.2006a)butusethelargerand timeofthedatarelease.Thequeryusedinthecatalogandin newer database of the SDSS DR6 and we apply improved thisstudyselectsallgalaxieswith:i)anaxialratioa/b>3 1 http://vizier.cfa.harvard.edu/viz-bin/VizieR?-source=J/A+A/445/765 2 http://casjobs.sdss.org/CasJobs www.an-journal.org (cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim 102 S.J.Kautsch:Fractionsofbulgelessandotherdiskgalaxies (a major axis, b minor axis) in the g-band, ii) an angular 1 majoraxisdiametera>30′′intheg-band,iii)colorsinthe 0.9 Sa(f), Sb(f) Sc(f), Scd(f), Sd(f), Irr(f) range of −0.5 < g −r < 2 and −0.5 < r −i < 2, iv) 0.8 amagnitudelimitintheg-band< 20mag.Theseselection d) 0.7 n criteriahavethefollowingforminSQL: ba 0.6 SELECT* ε (r- 0.5 into mydb.edge on dr6 catalog from 0.4 Galaxy as G 0.3 WHERE G.petroMag g < 20 0.15 and (G.isoA g/G.isoB g) > 3 0.1 and G.isoA g > 37.8 pixel thiscorrespondsto ε-e 0.005 anangularradiusof15′′ -0.05 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 and (G.dered g - G.dered r) between -0.5 e (r-band) e (r-band) and 2 Fig.1 This figure shows the relation between the luminosity and (G.dered r - G.dered i) between -0.5 weighted mean ellipticity ε (ordinate) and the adaptive elliptic- and 2 itye(abscissa)forallgalaxiesinthecatalogintheSDSSr-band. Thisqueryleadstoasampleof27308objects.Avisual Early-type disk galaxies are plotted inthe left part and late-type disksintherightpartofthisfigure.Thescatterplotsarepresented inspectionshowsanon-negligiblenumberofcontaminants. in the lower section of the figure. The finesolid lines in the up- Thecontaminantsaremostlystellarrefractionspikesandar- perdiagramsindicatethelinearrelationwithaslopeofone.The tifactssuchassatellite/meteortracks,emptyimagesaswell solidlinesinthebottomdiagramsshowthelocationofε−e=0. as face-on galaxies where elongated structures (e.g., bars, Thedashedlineinthescatterplotforthelatedisks(bottomright) spiralarms)simulateanedge-ondiskappearance.Inaddi- indicatesthederivedmeanoffsetbetweenεande. tion,someofthegalaxiesarealsoaffectedby“shredding” (Abazajianetal.2004).Thismeansthatasingleobjecthas 3 Morphologicalseparation more than oneuniquedetectionand thereforemultiple en- tries in the SDSS database. In the catalog, these contami- nants were removed manually. Because of the large num- Inthecatalogweusedanautomatedmorphologicalclassi- bers in the presentsample, this approachis inefficientand fication in orderto define six types of edge-ondisk galax- shouldbe quantified.We find thatthe majorityof the con- ies. These are galaxies with bulges (Sa(f), Sb(f)); sim- taminants are automatically removedby excludingthe ob- ple disk galaxies (Sd(f)); and disk-dominated intermedi- jectsflaggedwiththefollowingDR6PhotoFlags3: ate types (Sc(f), Scd(f)) as well as disky edge-on irregu- lars (Irr(f)).We followthe terminologyby de Vaucouleurs – “edge”indicatesgalaxiestruncatedonthesurveybor- (1959)wherespiralgalaxiesareflaggedwitha letterasso- ders; ciatedwiththeshapeofthespiralarms(ring-shapedgalax- – “saturated”indicatessaturatedpixels; ies havean“r” ands-shapedgalaxieshavean “s”).We in- – “notchecked” indicates that SDSS deblending may stead are using an “f” in brackets in order to indicate that beunreliable; thegalaxiescontainflatdisksseenedgeon. – “too few good detections” indicates objects Themorphologicalseparationinthecatalogisbasedon withnogoodcentroidfoundinallbands; the concentration index (CI) and the luminosity-weighted – “petroMagErr”>0.3ing, r, iindicatesthemagni- mean ellipticity of the elliptical isophotes (ε), both in the tudeerrors. SDSS r-band. The CI is a measure of the bulge size for edge-ongalaxiesandisdefinedastheratioofthePetrosian Thisleadsto aDR6 sampleof15176objects.We exclude radiithatcontain90%and50%ofthePetrosianfluxinthe alsotheobjectsthatdonothavePetrosianradii, samebandinacircularaperture.WeusedthePetrosianradii “petroR50 r”and“petroR90 r.”ThefinalDR6edge- thatarelistedinthe“GalaxyTable.”TheCIseparationval- ongalaxysamplecontains15127objects. uesdividetheearly(Sa(f),Sb(f))fromthelatetypes(Sc(f), We selectvariouslargerandomsamplesinordertoes- Scd(f),Sd(f)).Inaddition,lowCIvaluesseparatetheIrr(f) timate the remaining contribution of contaminants in this class from the late types. We use the same CI separation final sample. We find a rate of 5% of false detections for criteriaasinthecatalogbecausethePetrosianradiididnot the whole sample, mainly due to shredded galaxies, elon- undergochangesbetweenDR1andDR6. gatedHIIregionsindisks,projectedobjectsinspiralarms, ε is a discriminator of the flatness of edge-on disks emptyimages, projectedobjectsin halosofsaturatedstars andwasmeasureddirectlyontheindividualgalaxyimages andrarely,stellarspikesthatarestillpresentinthesample. of the galaxies in the catalog. This measurement was per- formedwiththeMIDASsurfphotpackage.Wefitelliptical 3 These flags are associated with every unique object in the isophotestothegalaxiesandderivedεastheweightedmean database andcontain important information ofthequality ofthe object: http://www.sdss.org/dr6/products/catalogs/flags.html. ellipticityofallisophotelevelsofanindividualgalaxy. (cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org Astron.Nachr./AN(2009) 103 We replacethemethodfindingεbyusingvariablesof- 50 fereddirectlyfromtheSDSStables.Thenewmethodoffers Sa(f) Sb(f) consistencywithin the SDSS parameters.Inaddition,with 40 thismethoditissimpletoreproduceourmorphologicalsep- arationcriteriainafastandefficientway.Diskflatnesscan beexpressedthroughtheadaptivemomentsprovidedinthe 30 er DR6“GalaxyTable.”Themomentsaremeasuredfromthe b m ellipticityandsizeoftheobjectswithintheSDSSpipeline4. Nu 20 The adaptive second moments from the SDSS (mE1 and mE2), 10 e+ = mE1, (1) 0 e× = mE2, (2) 0.2 0.3 0.4 0.5 0.6 e (r-band) can be convertedinto an “adaptive”ellipticity (e), using a Fig.2 The number distribution for the early-type disk galax- andbasmajorandminoraxis,respectively(c.f.,Vincent& iesoftheadaptiveellipticity(e)isshowninthishistogram.Sa(f) Ryden2005): galaxies,theclasscontaininggalaxieswiththelargestbulges,are drawnwithasolidline.Sb(f)galaxiesareindicatedwithadotted line. b v1−qe2++e2× e=1− =1−u . (3) Table 1 The limitingvalues of the adaptive ellipticity(e) and u a u1+ e2 +e2 concentration index (CI) between the morphological classes are t q + × listedinthistable.Thevaluesarevalidfor theSDSSr-band. In general,notethatthegalaxiesnearboundarieshavetheleastcer- Wenowcompareεwitheforallgalaxiesfromthecat- tainclassification. alog.TheupperpartofFig.1showstherelationbetweenε Class e CI andefortheearly-disktypes(left)andthelatetypes(right). ThediagramsonthebottomofFig.1showthescatterplots. LowerLimit UpperLimit LowerLimit UpperLimit Earlydisks(Sa(f),Sb(f))(ontheleftsideofthisplot)tend Sa(f) −− <0.40 ≥2.70 −− to have a roundershape (i.e., lower ellipticity values) of e Sb(f) ≥0.40 −− ≥2.70 −− comparedwith ε. In contrast, late disk typesSc(f), Scd(f), Sc(f) −− <0.766 ≥2.15 <2.70 Sd(f) and Irr(f) (shown in the right part of the figure) are Scd(f) ≥0.766 <0.816 ≥2.15 <2.70 flatterusingeinsteadofε.Wealsoseesomelate-typeout- Sd(f) ≥0.816 −− −− <2.70 liers having significantly lower ε compared to e. A visual Irr(f) −− <0.816 −− <2.15 inspectionoftheoutliersexhibitsthatmanyofthesepoints belongtomisclassifiedIrr(f)typesinthecatalog. 4 Results The limiting values of ε in table 1 in the catalog must berecalculatedinordertoseparatethegalaxiesintodiffer- 4.1 Fractionsofdifferenttypesofedge-ongalaxies entmorphologiesbasedone.Thelatetypesareconstantly shiftedfromasimplelinearrelationwithaslopeofonebe- We applythe new limiting valuesfromTable 1 to the new tweenεandeasshownintherightbottompartofFig.1.We DR6 edge-on galaxy sample. Figures 3, 4, 5, 6, 7, and 8 deriveameanoffsetofε−e=−0.016,indicatedasdashed show example images of galaxies with types Sa(f), Sb(f), lineinFig.1.Becausethelatedisksareflatterine,theoff- Sc(f), Scd(f), Sd(f) and Irr(f), respectively. These images setmustbeaddedtoεinordertoresemblethelimitingval- have an angular size of 100 square arcsec and north is to uesofthecatalog.Weemphasizethatthemainintentionof the top, east to the left. The images are downloaded from thisinvestigationistostudythefractionsofdisk-dominated theSDSSImageListTool5.Thenumberresultsareshown galaxies.Therefore,early-typedisks(classifiedasSa(f)and in Table 2.Inthis tablewe also reprintthenumbersof the Sb(f))arelessimportantforthispurpose.However,weuse classes found in the catalog on the right side. The largest ahistogramofthenumberdistributionofeforthegalaxies differencebetweenthecatalogandthisstudyisthefraction ofIrr(f).Theirincreasednumberinthepresentstudyisdue classified asSa(f)andSb(f)inthe catalog.With the aidof thishistogram(Fig.2)wedefineavalueofe = 0.4asthe tothecontaminationofmisclassifiedobjects,asrevealedin best separation value that resembles the dividing limit be- avisualinspectionofrandomlyselectedIrr(f)types.Aside tweenSa(f)andSb(f).ThelimitingvaluesforCIandeare fromthis,thefractionsofthedifferenttypesarealmostthe collectedinTable1. same.Forcompletenessconsiderationsoftheclasseswere- fertoSect.6inthecatalog. 4 http://www.sdss.org/dr6/algorithms/adaptive.html 5 http://cas.sdss.org/dr6/en/tools/chart/list.asp www.an-journal.org (cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim 104 S.J.Kautsch:Fractionsofbulgelessandotherdiskgalaxies Table2 Thegalaxyclassesandtheirfractionsareshowninthis table.Theabsolutenumbersofgalaxiesinthemainmorphological classes (Col. 2) and their percentages (Col. 3) are listed in this table.Forcomparisonthenumbersandpercentagesofthecatalog aregiveninCols.4and5. Database SDSSDR6 SDSSDR1 GeneralClass Number Percentages Number Percentages Sa(f) 966 6 222 7 Sb(f) 3993 26 843 26 Sc(f) 4835 32 1005 32 Fig.3 Sa(f): J111146.36+364442.3 (right panel), J143650.68 Scd(f) 2257 15 503 16 +294525.5(leftpanel). Sd(f) 2220 15 501 16 Irr(f) 856 6 95 3 Total 15127 100 3169 100 −17<M <−20) galaxies, their fraction is similar to B ours. The small difference of ∼4% is probably an effect of visualclassification: The “Tully Galaxy Catalog” is not ThelimitingvaluesforSd(f)ofthecatalogarebasedon restrictedtoedge-ongalaxies.Lessinclinedobjectscanex- fairly conservativeseparation criteria in order to minimize hibitcentrallightconcentrationsfromnuclei(e.g.,Walcher possible contamination from other classes. So the fraction et al. 2006) which are not seen edge-on. For this reason, of simple disks is 15% in the presentstudy. In the catalog someSdwouldhavebeenclassifiedasearliertypes. we show thatitisalso possible toincludethe Scd(f)types In Allen et al. (2006), 10095 galaxies with m <20 B inanextendedbulgelessdiskclass.Thefractionofthisex- fromthe“MillenniumGalaxyCatalog”(Liskeetal. 2003) tendedbulgelessdiskclass(Scd(f)andSd(f))is30%ofthe were morphologicallyanalyzedusing two componentSer- totalDR6sample.Thisvalueissimilartothevalueof32% sic spheroid + exponential disk decomposition (GIM2D, of the catalog.Therefore,the fractionof the extendedbul- Simard et al. 2002, 2008). The fraction of pure exponen- geless disk class in this work and the catalog corresponds tial disks is 14% and comparableto the fraction of simple toroughly1/3ofthetotalsampleofedge-ondiskgalaxies. disksinoursample. However, the contamination by other types is larger than The Neighboring Galaxy Catalog (Karachentsev et al. withthemorerigorousdefinedlimitsforsimpledisks. 2004) contains 77 disk-like objects among 451 galaxies with a magnitudelimitofMB >∼−12anda distance≤10 4.2 Comparisonwithotherstudies Mpc. This study exhibits an Sd fraction of 21±5%. This catalogincludeslow-luminositygalaxiesandthisincreases ItisgenerallyknownthatdiskgalaxiesoflateHubblemor- thecompletenessofthefractionofsimpledisksatthefaint phologiesare often bulgeless(e.g.,Hubble1936;Bo¨ker et end.Therefore,thesimplediskfractionislargercompared al.2002;Kormendy&Kennicutt2004).However,accurate totheDR6sample. numberstatisticsofthefractionofsimplediskgalaxiesare rare,probablyduetothelackoflargesamples. By comparing the simple disk fractions of this study with its progenitor catalogs, the catalog (Kautsch et al. Barazzaetal.(2008)studiedthefractionofbarsindisk 2006a) (16%) and the RFGC (Karachentsev et al. 1999) galaxies and they also analyzed the fraction of bulgeless (17%),alsorevealanexcellentagreementofthefractions. galaxiesbaseduponvisualinspection.Thefractionofbul- gelessdisksis∼20%intheirsample.Thesampletheyused This comparison shows a good agreement of the frac- is selected from the SDSS with −18.5≤M <−22 and tions of simple disk galaxies (Sd types) between different g redshifts between 0.01<z <0.03. Disk galaxies are se- studies which are using large numbers of analyzed galax- lectedusingacolorcut,andgalaxieswithinclinationslarger ies. The results of this comparison are summarized in Ta- than 60◦ are omitted. Their fraction of bulgeless galax- ble 3. Althoughthese studiesuse differentluminosity,dis- ies is close to that of the simple disks (Sd(f)) found here. tance, inclination and morphologicalselection criteria, the The ∼5%differencecan be explainedby consideringthat average fraction of simple disks is 16.2%± 3.2% among Barazza et al. (2008) used a color cut for the disk galaxy late-typegalaxies. selection. This could exclude red spiral types and there- foreslightlyoffsettheirsampletowardsbluerandlaterdisk 5 Summary anddiscussion types. Koda et al. (2007) used the “Tully Galaxy Catalog”6 This study presents the fractionsof disk-dominatedgalax- in order to select galaxies with morphological informa- ies and other edge-on disk galaxy types selected from the tion given in that catalog. The fraction of Sd galaxies is 11%. Considering the restriction to local (dist < 20h−1, SDSSDR6.Usingaquantitativemethodthatmeasuresthe diskflatnessandthebulgesize,thissampleisdividedinto 6 http://haydenplanetarium.org/universe/duguide/exgg tully.php morphologicalclassesofgalaxieswithbulges,intermediate (cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org Astron.Nachr./AN(2009) 105 Fig.8 Irr(f): J091028.24+071111.7 (right panel), J104210.63 Fig.4 Sb(f): J172506.84+565241.8 (right panel), J231006.72 +632430.5(leftpanel). –093953.6(leftpanel). Table3 Acomparisonofthefractionsofsimplediskgalaxies inthepresentworkandotherrecentstudiesisshowninthistable. Thefractions(inpercentages)arelistedinCol.1,andthemethod ofthemorphologicalclassificationinCol.2. Fraction[%] Method Thisstudy 15 Automated Kautschetal.2006a 16 Automated Barazzaetal.2008 20 Visual Kodaetal.2007 11 Visual Allenetal.2006 14 Automated Karachentsevetal.2004 21 Visual Karachentsevetal.1999 17 Visual Fig.5 Sc(f): J020800.74–082442.1 (right panel), J143917.88 +200439.7(leftpanel). typesandapparentlybulgelessobjects.Eachofthesebroad typescontainsroughly1/3ofthetotalsample.Furthersub- divisionisappliedtothesetypesandallowsustodefinethe fractionofsimplediskgalaxies,Sd(f),tobe15%oftheto- taldiskgalaxysample.Thefractionsofothermorphological Hubbletypesofdisk galaxiesarealso presentedand com- paredtothecatalogofedge-ondiskgalaxies(Kautschetal. 2006a). We also presenta comparisonofthefrequencyofsim- ple disk galaxies from various galaxy catalogs and find a simple diskfractionof∼16%±3%onaverage.The frac- tionsarearobustresultbecauseoftheexcellentagreement betweenthestudies,althoughvariouscriteriawereusedto Fig.6 Scd(f): J225828.27–103332 (right panel), J124943.24 selectthegalaxies.SmalldifferencesoftheSdfractionscan +044610.1(leftpanel). be explained qualitatively. We conclude that simple disks are a common galaxytype, althoughformationand evolu- tion models are challenged by explaining bulgeless galax- ies.Hence,acomprehensivedescriptionofsimpledisksre- mainstobeexplored. Acknowledgements. The author expresses his gratitude to the anonymous referee and to Dr. Fabio D. Barazza, Prof. Anthony H.GonzalezandLeahE.Simonforthesupportofthisworkand criticalreadingofthemanuscript.Theauthoralsowantstothank Prof.EvaK.GrebelandProf.JayS.Gallagherfortheirkindsup- port. Funding for the SDSS (http://www.sdss.org/) and SDSS-II hasbeenprovidedbytheAlfredP.SloanFoundation,thePartic- ipatingInstitutions,NSF,theU.S.DepartmentofEnergy,NASA, Fig.7 Sd(f): J214439.43–064122.5 (right panel), J104658.44 theJapaneseMonbukagakusho, andtheMaxPlanckSociety,and +382728(leftpanel). theHigherEducationFundingCouncilforEngland.TheSDSSis managed by the Astrophysical Research Consortium (ARC) for www.an-journal.org (cid:13)c 2009WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim 106 S.J.Kautsch:Fractionsofbulgelessandotherdiskgalaxies theParticipatingInstitutions.TheParticipatingInstitutionsarethe Karachentsev, I.D., Karachentseva, V.E., Huchtmeier, W.K., AmericanMuseumofNaturalHistory,AIP,Univ.ofBasel,Univ. 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